In many industrial processes using a gaseous feed stream it is desirable or necessary to remove carbon dioxide from the gaseous feed stream prior to certain steps of the process. For example, in the separation of atmospheric air into its component parts by cryogenic distillation, it is necessary to prepurify the air by removing carbon dioxide and water vapor from the air feed prior to refrigerating the air; otherwise, these gases would condense and freeze in the refrigeration heat exchange equipment and eventually clog the equipment, thereby necessitating removal of the equipment from service for removal of the frozen carbon dioxide and ice.
The carbon dioxide and water vapor can be removed from gas streams by a number of techniques. One well known method involves the use of pairs of reversing heat exchangers that are operated alternately, such that one heat exchanger is in purification service while the other is undergoing frozen carbon dioxide and ice removal. Specifically, in this method the gas feed is passed through one heat exchanger in exchange with a refrigerant, which causes the carbon dioxide and water vapor to freeze onto the surfaces of the heat exchanger. When the buildup of frozen carbon dioxide and ice in the heat exchanger reaches a certain level, the heat exchanger is taken out of service to remove, by sublimation and melting, the frozen carbon dioxide and ice. The other heat exchanger of the pair, from which frozen carbon dioxide and ice have been removed, is then placed into purification service. This method has the disadvantage that a considerable amount of heat energy is required to sublime and melt the frozen carbon dioxide and ice during regeneration of the heat exchangers.
A popular method of removing carbon dioxide and water vapor from gas streams is adsorption. One common adsorption method of air prepurification is PSA using two serially-connected adsorption layers, the first layer containing a desiccant, such as silica gel or activated alumina for water vapor removal, and the second layer containing a carbon dioxide-selective adsorbent, such as sodium-exchanged type X zeolite (13X zeolite). A two layer air prepurification system comprising a first bed of adsorbent selective for the removal of water from an air stream, for example alumina or silica gel, and a second bed of adsorbent selective for the removal of carbon dioxide, for example, 5A, 13X, calcium X or sodium mordenite, is disclosed in U.S. Pat. No. 4,711,645. Other two layer air prepurification PSA processes are described in U.S. Pat. Nos. 5,110,569 and 5,156,657, the disclosures of which are incorporated herein by reference. This method has a number of disadvantages. Firstly, it is difficult to desorb carbon dioxide from the 13X zeolite. Also, the layer of zeolite develops "cold spots" in its upstream region, i.e. in the vicinity of the inlet of the zeolite adsorbent, and the process becomes unstable with time.
Temperature swing adsorption (TSA) processes have also been practiced for the removal of carbon dioxide from nonpolar gas streams using the above discussed combination of adsorbent layers. U.S. Pat. No. 5,110,569, mentioned above, shows such a process. TSA processes have also been practiced using a single layer of adsorbent. A major disadvantage of the described TSA process is that a great quantity of heat energy is required in the adsorbent regeneration step, since both layers must be heated sufficiently to drive off the adsorbed moisture and carbon dioxide.
Air prepurification by PSA has also been practiced using a single bed of adsorbent which removes both water vapor and carbon dioxide. Such a process is disclosed in U.S. Pat. No. 5,232,474, which uses a single layer of activated alumina as adsorbent. The principal disadvantages of this method of air prepurification are that it is difficult to produce high purity air by this method, a high volume of purge gas is required to effect adequate adsorbent regeneration and the process becomes unstable over time.
Certain metal-exchanged type Y zeolites have been used to remove ethylene from air streams. For example, EP 0 750 852, published Jan. 2, 1997, discloses the removal of water vapor, ethylene, nitrogen and carbon dioxide from air streams by adsorption using a series of adsorption beds, including a first bed containing alumina to remove moisture from the air, a second bed containing silver-exchanged type Y zeolite to remove ethylene from the air and a third bed containing 13X zeolite to remove nitrogen and carbon dioxide from the air.
Superior methods of producing high purity air are continuously sought. The present invention provides a method which accomplishes this, and does so with low energy and capital expenditures.